1. Field of the Invention
The present invention relates generally to a beamforming apparatus and a method in a smart antenna system, and in particular, to an apparatus and method for performing fixed beamforming by estimating a Direction Of Arrival (DOA) with a maximum Signal-to-Interference and Noise Ratio (SINR) in a smart antenna system.
2. Description of the Related Art
The performance and capacity of a mobile communication system are limited due to the features of radio channels such as co-channel interference, path loss, multipath fading, signal delay, Doppler spread, and shadowing. Hence, a mobile communication system mitigates the performance and capacity limitations through power control, channel coding, rake reception, antenna diversity, sectorization, frequency division, and spectrum spread.
With the advent of the wireless multimedia era, the demands for high-speed transmission of a large volume of data on a radio channel are rapidly increasing. The development of mobile communication technology has brought about a hybrid cell environment with different service signals. Under the hybrid cell environment, a transmitted signal is strongly interfered with by other signals with a relatively great transmit power and a relatively wide bandwidth. As a solution to performance degradation caused by the interference and the channel characteristics, a smart antenna system has attracted much attention.
FIG. 1 illustrates beam patterns in a general smart antenna system. Referring to FIG. 1, in the smart antenna system, a Base Station (BS) 100 forms a beam pattern 111 for a pilot signal such that it covers an entire cell service area 113. Also, the BS 100 estimates a channel for a Mobile Station (MS) 102 and beamforms a traffic signal for the MS 102 in a beam pattern 115 according to the channel estimation. Since beams are narrowed in the beam pattern 115, transmit power is reduced.
Accordingly, the smart antenna system can increase the SINR of a transmitted signal by directing a beam in the direction of an intended MS. Adaptive beamforming and fixed beamforming are two beamforming schemes are considered for the smart antenna system. In adaptive beamforming, a beam coefficient is generated adaptively according to the channel status of the MS and beamforming is performed with the beam coefficient, whereas in the fixed beamforming, a fixed antenna beamforming coefficient is used for beamforming.
As illustrated in FIG. 2, a predetermined fixed antenna beamforming coefficient is used for beamforming in the fixed beamforming scheme.
A spatial beam is selected which maximizes SINR, taking into account the fixed beam coefficient, the location of the MS, and spatial signal characteristics. Also, the DOA of the MS is estimated and the beam is steered in the DOA. As illustrated in FIG. 2, the beam is steered in an optimum direction using a predetermined beam coefficient.
For accurate DOA estimation, Multiple SIgnal Classification (MUSIC) and Estimation of Signal Parameter via Rotational Invariance Technique (ESPRIT) are generally used. However, these DOA estimation methods are not effective in accurately estimating the DOA of a main signal in case of multiple paths. In addition, computational complexity increases with the number of antennas, which makes it difficult to implement the conventional DOA estimation methods in typical communication systems.
Depending on the environment of a communication system adopting the fixed beamforming scheme, antenna paths differ in cable length. As a result, predetermined beamforming coefficients are useless.